In general, in an electrophotographic image forming apparatus, the quality of an image formed on a photoreceptor is improved by controlling an image forming process such as charging to a photoreceptor, exposure, and development. There is an apparatus in which the control of this image forming process is performed based on detection results from a detection sensor to detect the surface potential of the photoreceptor. Such a detection sensor detects the surface potential of the photoreceptor by, for example, a detection electrode in which electric charge is induced by the electrostatic induction of the photoreceptor. In order to prevent the electric charge induced in the detection electrode from being influenced by an external electric field, the detection electrode is covered with a metal shield case having a detection window. When scattering toner or the like enters the metal shield case, there is a fear that erroneous detection occurs, or the detection sensor is damaged.
Thus, hitherto, it is general that the surface potential detecting sensor is disposed at the upstream side of the developing process where there is less influence of the scattering toner.
However, in a tandem-type image forming apparatus including respective process units of charging, developing and the like which act on plural photoreceptors, miniaturization of the process units is required. Thus, in the tandem-type image forming apparatus, due to limitation in layout, there is a case where the detection sensor must be disposed at the downstream side of the developing process, where the influence of scattering toner is liable to occur. In this case, it is necessary to provide a shutter on a detection window in order to prevent that the scattering toner enters the metal shield case through the detection window of the detection sensor, and erroneous detection occurs or the detection sensor is damaged. However, when the shutter for opening and closing the detection window of the shield case is provided, there is a fear that the charging state of the shutter exerts a large influence on the detection sensor, output variation occurs in the detection electrode, and the surface potential of the photoreceptor is erroneously detected.
Then, it is desired to develop a surface potential detecting device of an image forming apparatus, which can accurately detect the surface potential of the photoreceptor without being influenced by a shutter.